The present disclosure relates to a sheet feeder for picking up and feeding sheets one by one and an image processing apparatus using this sheet feeder.
Conventionally, there is known an auto document feeder (ADF) applied to an image processing apparatus such as a copier or a facsimile machine. This auto document feeder includes a document tray on which a stack of documents is placed and a pickup roller for successively picking up the documents on this document tray from the uppermost one and feeding them into a feeder main body. A document image reading process is successively applied to each sheet for the documents fed by the pickup roller.
To pick up the documents by the pickup roller, the pickup roller needs to be held in contact with the upper surface of the uppermost document. However, if the pickup roller is present at a position where the pickup roller can be held in contact with the upper surface of the uppermost document, it stands as a hindrance when a user sets new documents on the document tray. Accordingly, there is known an image reading apparatus configured to retract a pickup roller upward upon the elapse of a predetermined time after a document reading operation is finished so that a user can easily set next documents on the document tray.
In some cases, a motor for generating a drive force for retracting the pickup roller upward is desirably also used to drive another member. Thus, an electromagnetic clutch may be provided to interrupt connection between the motor and a mechanism for retracting the pickup roller upward. However, if the electromagnetic clutch is disengaged in a state where the pickup roller is retracted upward by the drive force of the motor as described above, the pickup roller hangs down under its own weight and stands as a hindrance when a user sets document. On the other hand, there has been an inconvenience that power consumption increases if the electromagnetic clutch is kept engaged.